Thin-film transistor (TFT) devices are conventionally fabricated on glass or other silicon-based substrates using a sequence of deposition, patterning, and etching steps. For example, amorphous silicon TFT devices require deposition, patterning, and etching of metals, such as aluminum, chromium or molybdenum; of amorphous silicon semiconductors; and of insulators, such as SiO2 or Si3N4, onto a substrate. Fabrication facilities for LCD panels are set up to handle glass substrates that are generally in the 500 to 700 um thickness range. The semiconductor thin film is formed in layers having typical thicknesses ranging from several nanometers (nm) to several hundred nm, with intermediary layers having thicknesses on the order of a few microns. The semiconductor thin film may be formed over an insulating surface that lies atop the rigid substrate.
The requirement for a rigid substrate has been based largely on the demands of the fabrication process itself. Rigidity allows the fabrication system to more accurately register the substrate in position for the different process steps. Thermal characteristics are also particularly important. TFT devices are fabricated at relatively high temperatures, making it difficult to work with many types of plastics and with some metals, due to thermal expansion characteristics. Thus far, the range of substrate materials that have been used successfully is somewhat limited, generally to glass, quartz, or other rigid, silicon-based materials.
For some applications there would be an advantage in fabricating TFT devices on thin metal foils that could provide a thinner and more rugged substrate than glass. Such foils could have thicknesses in the range of 50 to 250 um or greater. One method to accommodate such foils in a glass-based fabrication facility is to laminate the foil substrate to a glass carrier for the fabrication process, then delaminate it for use once fabrication is completed. However, lamination in this manner presents a number of difficulties, for example:
(i) Dimensional instability of the laminated stack of materials over the temperature range. The different materials used for the foil substrate, carrier, and lamination are subjected to a wide range of temperatures during fabrication. In some cases, the total temperature change during various phases of fabrication can be around 250 degrees C. Over this changing temperature range, mismatches of their respective Coefficients of Thermal Expansion (CTE) can cause excessive stress along the interface between the different materials. This applies to the laminated materials themselves as well as to materials used to form the TFT devices. In some cases, this can cause the laminated materials to separate or stretch or can even crack the glass carrier.
(ii) Failure to maintain substrate flatness and position throughout processing. In order lie flat in position against the carrier throughout processing, the laminate material that acts as the adhesive must be applied uniformly between the foil substrate and the carrier and must work well over the complete temperature range. The substrate must be maintained in positional register so that it does not shift relative to the carrier surface during processing. Air pockets must be prevented from forming within the metal foil-glass carrier stack.
(iii) The need for proper preparation and conditioning of the foil surface. TFT fabrication requires that the substrate surface be extremely smooth, with no more than about 50 nm peak-to-peak roughness. Handling and preparatory steps must be performed with this stringent requirement in mind. This includes steps for lamination without damaging the foil surface.
(iv) Delamination without damaging the circuitry, substrate, or carrier. Once fabrication is complete, it is important that the treated substrate be removed from the carrier without stretching, tearing, or otherwise damaging the material or causing stress to the TFT components. It can be further advantageous to be able to re-use the carrier.
(v) Chemical incompatibility of laminated materials. Materials laminated together must be chemically compatible with each other as well as with the chemical processes that are used in TFT circuit fabrication.
(vi) Poor signal handling and isolation with respect to the metal substrate. Planarization and other methods must be used to prevent shorting to the conductive substrate. Other considerations related to signal handling can include capacitive coupling.
If a separate lamination sheet is assembled into the metal and glass wafer stack, it is desirable to easily trim such a lamination sheet to the edge of the metal foil after assembly to the glass and before heating. As can be appreciated, the task of accurately registering a thin precut 12 to 50 um thick plastic lamination sheet to the thin metal wafer is extremely difficult.
In addition to metal foils, there can also be advantages in circuit fabrication onto thin plastic substrates or other non-metallic films. To use such materials, some type of carrier is needed to provide support during fabrication. The same general requirements given earlier in (i)-(v) apply for providing non-metallic substrates on a suitable carrier for TFT fabrication processing.
There is an interest in TFT fabrication onto a metal foil and other substrates using a glass carrier. However, the problems listed above (in (i)-(vi)) present a challenge to efforts for achieving large-scale production fabrication onto metallic or non-metallic foil substrates.